The present invention relates to a dielectric filter. More specifically, it relates to a dielectric filter used as a filter in the microwave band.
Personal radio telephones, mobilephones and similar devices operating on microwaves use dielectric filters such as those disclosed, for example, in Japanese Laid-Open Patent Application Nos. 15401/1986 and 192101/1986, and Japanese Laid-Open Utility Model Application No. 153703/1989.
FIG. 20 shows one example of a conventional dielectric filter.
The dielectric filter comprises a filtering member 100 composed of three dielectric resonators 100a, 100b and 100c joined to one another, a substrate 101 having capacitive elements formed thereon, and a case 102 covering the filtering member 100 and the substrate 101.
Each of the dielectric resonators 100a to 100c has a through-hole 103 extending from its exposed end alongside the substrate 101 to its short-circuited opposite end. An inner conductor 104 is formed on the inner surface of the through-hole 103, and an outer conductor 105 is formed on the outer peripheral surface, excluding the exposed end surface, of the filtering member 100.
Capacitive elements connected to the respective inner conductors of the resonators 100a to 100c are formed on the substrate 101. The capacitive elements are formed by conductive platings 106, 107 and 108, formed on the upper surface of the substrate 101, and conductive platings 109 and 110 (see broken lines in FIG. 21) formed on the reverse surface thereof. In particular, the conductive platings 106, 107 and 108 are patterned so as to include inter-extensive sections. Capacitance K is effected between the inter-extensive sections thus coupling the resonators. In addition, input/output capacitive couplings C.sub.IN and C.sub.OUT to the resonators are formed between the conductive platings 106 and 108 on the upper surface of the substrate, and the respectively corresponding conductive platings 109 and 110 opposite on the reverse surface.
The conductive platings 106 to 108 on the surface of the substrate 101 are respectively connected to the inner conductors 104 of the corresponding resonators 100a to 100c by, for example, conductive wires 11.
The conductive platings thus forming the capacitive elements of such a conventional dielectric filter are superficially exposed. Consequently, electric field leakage arises between the adjacent conductive plating pairs 106/107, and 107/108, such that the effective dielectric constant decreases. In order to obtain a predetermined coupling capacitance K, therefore, the size of the conductive platings on the surface of the substrate must be large enough to compensate, limiting the extent to which the size of the substrate, and therefore of the entire dielectric filter, can be reduced.
Furthermore, if conductive foreign matter, for example, excess solder in attaching a case 102, or pieces of the outer conductor 105 which may break off in tuning the resonators, lodges between the conductive platings on the surface of the substrate, desired coupling capacitance will not obtain.
In a different approach, the aforementioned Japanese Laid-Open Patent Application No. 192101/1986 discloses a dielectric filter of construction wherein conductor and ground electrode platings are formed on the surface of a substrate, and resonator inner conductors are connected to respective conductor platings. In this construction, capacitive coupling between the resonators is effected between adjacent conductor platings, and a frequency-adjusting capacitance is effected between the conductor and the ground electrode platings. The formation of frequency-adjusting capacitance allows the resonators to be of decreased length.
Also in this construction, however, capacitive elements are formed in the superficial patterning on the substrate. Consequently, electric field leakage similarly arises between the platings, such that the effective dielectric constant decreases.
Furthermore, in the foregoing conventional construction, since the conductive platings are formed superficially on the substrate, the capacitive elements among the conductive platings are liable to be affected by stray capacitances, hindering the attainment of desired filter characteristics. To address this problem, Japanese Laid-Open Utility Model Application No. 153703/1989 discloses a dielectric filter of construction wherein a ground electrode plating includes extensions which separate from one another conductor platings connected to respective resonator inner conductors. However, with this construction it also is not possible to overcome, satisfactorily, the adverse effects of stray capacitances.
A dielectric filter constructed such that a microstrip line is formed between conductive platings on the surface of a substrate is further known conventionally. A plurality of resonators are electrically connected to one another by this microstrip line.
However, since the plating constituting the microstrip line is exposed on the substrate, electric field leakage arises, particularly around patterned features of the microstrip, decreasing the effective dielectric constant. The electrical equivalent length of the microstrip must be increased, therefore, the actual length of the patterned microstrip is increased. Consequently, this limits the extent to which miniaturization of the microstrip patterned plating, and thus of the substrate, is possible.
Moreover, if conductive foreign matter sticks to, or is otherwise brought into contact with the microstrip line, coupling capacitances will fluctuate widely. Wherein no ground electrode plating is formed on the reverse surface of the substrate, the microstrip line develops inductance through its patterned arrangement. Also in this case, however, the coupling capacitances fluctuate widely wherein conductive foreign matter lodges on the microstrip line.